Discs of disc brakes for two-wheeled vehicles such as motorcycles and bicycles (slide sections for brake pads) can be repeatedly heated to about 500° C. during braking by friction heat with brake pads. Accordingly, materials used for brake discs require heat resistance (temper softening resistance) sufficient not to be softened by heat generated during braking
An excessively hard brake disc, on the other hand, squeals during braking and considerably wears a brake pad. There is therefore an appropriate range of hardness for brake discs, typically from about 31 to 38 in HRC (Rockwell Hardness C-scale). The hardness, however, can be permitted to a level exceeding HRC 40 because the upper limit thereof varies with the type of brake pad and the combination of the brake pad and the disc.
Brake discs also require superior corrosion resistance (rusting resistance) for esthetic considerations and concern about an adverse effect on brake performance. Accordingly, low-carbon martensitic stainless steels containing 12% to 13% by mass of chromium have been frequently used as materials for brake discs because they not only have the corrosion resistance required for brake discs, but also have appropriate hardness in an as-quenched state and maintain substantially appropriate hardness when tempered at 500° C. for about one hour.
However, still superior heat resistance has been demanded for brake discs and materials therefor in view of improving brake performance such as braking capacity, reducing weight, and diversifying design. To meet that demand, various highly heat-resistant steels have been proposed. For example, Japanese Unexamined Patent Application Publication Nos. 2001-220654 and 2007-070654 disclose steels, with superior temper softening resistance, produced by adding, or increasing the amounts of, elements that improve temper softening resistance such as carbon, copper, niobium, vanadium, and molybdenum so that it maintains a hardness of HRC 30 or more not only after quenching, but also after tempering at 550° C. to 650° C. for about one hour.
In addition, Japanese Unexamined Patent Application Publication No. 2005-307346 discloses a steel produced by adding appropriate amounts of niobium, nickel, and vanadium and increasing the nitrogen content to relatively reduce the carbon content so that it has superior corrosion resistance, has an appropriate quenching hardness, namely, HRC 32 to 38, and maintains a high hardness, namely, HRC 32 or more, after tempering at 600° C. for two hours.
In general, brake discs are rarely heated to a temperature range of 650° C. to 700° C. during braking of, for example, a motorcycle or a bicycle. Nevertheless, a material for brake discs that has heat resistance in such a temperature range provides advantages including improved brake performance, a weight reduction due to reduced thickness, and greater design flexibility. These advantages are particularly beneficial for brake discs of large- and medium-sized motorcycles, especially sport-style motorcycles, and accordingly an increase in the heat resistance of the materials therefor has been highly expected.
Accordingly, it could be helpful to provide a material for brake discs that has a higher heat resistance (temper softening resistance) than conventionally used or proposed materials. It could also be helpful to provide a material for brake discs that has a hardness after quenching of HRC 31 to 40 and temper softening resistance sufficient to maintain an appropriate hardness, namely, HRC 31 to 38, after tempering at 700° C. for one hour.